Conventionally, semiconductor ceramic compositions prepared by adding various semiconducting elements to a BaTiO3-based oxide have been proposed as a material showing PTC (Positive Temperature Coefficient of resistivity) characteristic. The semiconductor ceramic composition can be used as a PTC element by providing electrodes thereon.
Most of semiconductor ceramic compositions composed of a BaTiO3-based oxide have a Curie temperature of around 120° C. In these semiconductor compositions, the Curie temperature needs to be shifted according to use. For example, it has been proposed to shift the Curie temperature by adding an SrTiO3-based oxide to a BaTiO3-based oxide, but in this case, the Curie temperature is shifted only in a negative direction and is not shifted in a positive direction. Out of materials used in practice at present, the material known as an additive capable of shifting the Curie temperature in a positive direction is PbTiO3. However, since lead is an element causing environmental pollution, a lead-free semiconductor ceramic material containing no lead is demanded.
As the production method for a lead-free semiconductor ceramic composition, there has been proposed a method for producing a BaTiO3-based semiconductor ceramic composition, in which at least one of Nb, Ta and rare earth elements is added to a composition which is represented by a composition formula of Ba1-2X(BiNa)xTiO3 in which a portion of Ba in a BaTiO3-based oxide is substituted by Bi—Na, in which x satisfies 0<x≦0.15, followed by subjecting the composition to calcination in nitrogen and then sintering in an oxidative atmosphere (Patent Document 1).
According to the production method described in Patent Document 1, the temperature coefficient of resistance as one of PTC characteristics can be prevented from decreasing. Also, in Examples of Patent Document 1, all of raw materials Ba, Ti, Bi and Na in the composition formula above are mixed at a time and then calcined.
In addition, as the production method for a lead-free semiconductor ceramic composition, a semiconductor ceramic composition having a crystal grain which is represented by a composition formula of [(BiNa)x(Ba1-yRy)1-x]TiO3, wherein a center portion and an outer shell portion of the crystal grain are different from each other in composition has been proposed (Patent Document 2). In Patent Document 2, the semiconductor ceramic composition containing the crystal grain above is said to have an effect of increasing the amount of Schottky barrier formation and enhancing the temperature coefficient α of resistance. Patent Document 2 discloses a method of separately preparing a (BaQ)TiO3 calcined powder (in which Q is a semiconducting element) and a (BiNa)TiO3 calcined powder, and then mixing the powders, followed by forming and sintering the mixture.
Patent Document 3 describes a lead-free semiconductor ceramic composition and discloses a compositional formulation where the main component is a BamTiO3-based composition having a perovskite structure represented by formula AmBO3 and a portion of Ba constituting the A site is substituted by at least an alkali metal element, Bi, and a rare earth element. In Examples of Patent Document 3, there is disclosed a semiconductor ceramic composition having a main phase represented by the composition formula of (Ba0.898Na0.05Bi0.05Y0.002)mTiO3+0.00025Mn, in which a Y raw material is added.